CS 241 Discussion Section 111011 Contents Signals MP

CS 241 Discussion Section (11/10/11)

Contents Signals MP 7 - HTTP Headers

Review: signals Asynchronous notification to a process indicating some action should be taken Sending signals to a process: kill -<signal> <pid> int kill(pid_t pid, int sig); We can signal individual threads, too: int pthread_kill(thread_t tid, int sig);

What can we do with signals? • Handle them! – Default or Ignore – Custom function with sigaction • Block them! – Delay delivery with masks – Then we can sigwait to get them.

Lots of signal functions #include <signal. h> int sigemptyset(sigset_t *set); int sigfillset(sigset_t *set); int sigaddset(sigset_t *set, int signo); int sigdelset(sigset_t *set, int signo); int sigismember(const sigset_t *set, int signo); int sigprocmask(int how, const sigset_t *restrict set, sigset_t *restrict oset) int sigaction(int signo, const struct sigaction *act, struct sigaction *oact); int sigwait(const sigset_t *restrict sigmask, int *restrict signo);

Process Signal Masks Setting SIGINT to be blocked if ((sigemptyset(&set) == -1) || (sigaddset(&set, SIGINT) == -1)) perror(“Failed init signal set”); else if (sigprocmask(SIG_BLOCK, &set, &oldset) == -1) perror(“Failed to block SIGINT”); SIG_BLOCK adds set to current mask oldset will store the previous signal mask

Thread Signal Masks pthread_sigmask(): Takes same parameters as sigprocmask Only affects the signal mask of a single thread Signal mask is inherited on thread creation

Signal Handlers Allow us to change what happens when a signal is received void handler(int signo) { … } struct sigaction act; act. sa_flags = 0; act. sa_handler = handler; // additional signals blocked in the handler sigemptyset(&act. sa_mask); sigaction(SIGUSR 1, &act, NULL); sa_handler can also be set to SIG_DFL (default) or SIG_IGN (ignore)

sa_handler vs. sa_sigaction We can get additional information about the signal void handler(int signo, siginfo_t* info, void* context); act. sa_flags = SA_SIGINFO; // fill sa_sigaction instead of sa_handler act. sa_sigaction = handler; Extra information contains, e. g. , the source of the signal (info>si_code): SI_USER – user-created signal (with abort, kill, etc. ) SI_TIMER – a POSIX: RTS timer expired etc.

One function to rule them all void ( *signal(int signum, void (*handler)(int)) ) (int); But what does it all mean?

One function to rule them all Symbol void ( *signal(int signum, void (*handler)(int)) ) (int); But what does it all mean?

One function to rule them all Symbol Function Args void ( *signal(int signum, void (*handler)(int)) ) (int); But what does it all mean?

One function to rule them all Symbol Function Args void ( *signal(int signum, void (*handler)(int)) ) (int); Return Type But what does it all mean?

One function to rule them all void ( *signal(int signum, void (*handler)(int)) ) (int); But what does it all mean? typedef void (*sighandler_t)(int); sighandler_t signal(int signum, sighandler_t handler);

Code Examples ds 12/signals. c ds 12/http. c